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std::thread::thread

From cppreference.com
< cpp‎ | thread‎ | thread
 
 
Thread support library
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thread();
(1) (since C++11)
thread( thread&& other );
(2) (since C++11)
template< class Function, class... Args >
explicit thread( Function&& f, Args&&... args );
(3) (since C++11)
thread(const thread&) = delete;
(4) (since C++11)

Constructs new thread object.

1) Creates new thread object which does not represent a thread.
2) Move constructor. Constructs the thread object to represent the thread of execution that was represented by other. After this call other no longer represents a thread of execution.
3) Creates new std::thread object and associates it with a thread of execution. The new thread of execution starts executing
std::invoke(decay_copy(std::forward<Function>(f)), decay_copy(std::forward<Args>(args))...);

where decay_copy is defined as

template <class T>
std::decay_t<T> decay_copy(T&& v) { return std::forward<T>(v); }
Except that the calls to decay_copy are evaluated in the context of the caller, so that any exceptions thrown during evaluation and copying/moving of the arguments are thrown in the current thread, without starting the new thread.
The completion of the invocation of the constructor synchronizes-with (as defined in std::memory_order) the beginning of the invocation of the copy of f on the new thread of execution.
This constructor does not participate in overload resolution if std::decay_t<Function> is the same type as std::thread.
(since C++14)
4) The copy constructor is deleted; threads are not copyable. No two std::thread objects may represent the same thread of execution.

Contents

[edit] Parameters

other - another thread object to construct this thread object with
f - Callable object to execute in the new thread
args... - arguments to pass to the new function

[edit] Postconditions

1) get_id() equal to std::thread::id() (i.e. joinable is false)
2) other.get_id() equal to std::thread::id() and get_id() returns the value of other.get_id() prior to the start of construction
3) get_id() not equal to std::thread::id() (i.e. joinable is true)

[edit] Exceptions

1-2)
noexcept specification:  
noexcept
  
3) std::system_error if the thread could not be started. The exception may represent the error condition std::errc::resource_unavailable_try_again or another implementation-specific error condition.

[edit] Notes

The arguments to the thread function are moved or copied by value. If a reference argument needs to be passed to the thread function, it has to be wrapped (e.g. with std::ref or std::cref).

Any return value from the function is ignored. If the function throws an exception, std::terminate is called. In order to pass return values or exceptions back to the calling thread, std::promise or std::async may be used.

[edit] Example

#include <iostream>
#include <utility>
#include <thread>
#include <chrono>
#include <functional>
#include <atomic>
 
void f1(int n)
{
    for (int i = 0; i < 5; ++i) {
        std::cout << "Thread 1 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}
 
void f2(int& n)
{
    for (int i = 0; i < 5; ++i) {
        std::cout << "Thread 2 executing\n";
        ++n;
        std::this_thread::sleep_for(std::chrono::milliseconds(10));
    }
}
 
int main()
{
    int n = 0;
    std::thread t1; // t1 is not a thread
    std::thread t2(f1, n + 1); // pass by value
    std::thread t3(f2, std::ref(n)); // pass by reference
    std::thread t4(std::move(t3)); // t4 is now running f2(). t3 is no longer a thread
    t2.join();
    t4.join();
    std::cout << "Final value of n is " << n << '\n';
}

Possible output:

Thread 1 executing
Thread 2 executing
Thread 1 executing
Thread 2 executing
Thread 1 executing
Thread 2 executing
Thread 1 executing
Thread 2 executing
Thread 2 executing
Thread 1 executing
Final value of n is 5

[edit] References

  • C++11 standard (ISO/IEC 14882:2011):
  • 30.3.1.2 thread constructors [thread.thread.constr]